The rapid increase in the operating speed of computers has led to significant demands on both data storage capacity and access speed. The hard disk drive has been steadily evolving and continues to offer a cost-effective solution to both capacity and speed requirements. Increases in data storage densities and miniaturization of disk drives have enabled even small portable computers to have access to large amounts of on-line disk storage.
In disk drives, head-to-media speeds are such that an air bearing is generated between the head and the disk. At increased speeds the air bearing increases. Thus, without means to counter the tendency to increase the head/media spacing, losses will occur.
Manufacturers of hard disk drives typically measure the flying height of all head/gimble assemblies before assembling them into drives in order to avoid reworking drives after assembly when they do not meet specifications. While head/media spacings in excess of 1 micron can be measured using monochromatic fringe counting techniques, spacings below micron are measured using white light interferometry due to its greater resolution in the range of 250 to 750 nm. Constructive or destructive interference results in the generation of different color fringes which are compared to a Newton's color chart or analyzed by spectrometer. This technique is the current industry standard, however, at spacings of less than 150 nm, the colors wash together and cannot be interpreted with reasonable accuracy.
Various other techniques have been developed for the measurement of small spacings, however, there methods are still incapable of accurately measuring spacing down to contact. One example is U.S. Pat. No. 4,593,368 by Fridge et al. This patent describes the use of a computerized spectro-photometer to analyze the color of white light fringes produced at the interface of two surfaces, one of which is transparent, when subject to broad-band illumination. This measuring system and technique has the following disadvantages: 1) At very low spacing (less than .lambda./5) no distinct colored fringes are produced. Therefore, at this small spacing, relatively small changes in light intensity as a function of wavelength are measured by the computerized spectrophotometer. Since the measurable change in intensity as a function of wavelength (the color) is greatly reduced at spacings below .lambda./5, the signal-to-noise ratio of the measurement greatly decreases for such small spacings. 2) The spectro-photometer employed requires 0.05 seconds to acquire the intensity data for the spectrum of light being used for the measurement. The lengthy time required for data acquisition precludes dynamic measurement of spacing above 10 Hz.
In a method disclosed by Tanaka et al. (U.S. Pat. No. 4,630,926) an interferometer is used to dynamically measure head/disk spacing. A xenon light source with a monochrometer produces monochromatic light which is directed over the length of a slider which is inclined such that the spacing between a clear glass disk and the slider varies by more than .lambda./4. In such a case where the slider is incident with respect to the disk and the spacing varies by more than .lambda./4, at least one maxima and one minima of interferometric fringe intensity occurs. Tanaka et al. teach that at the maximum and minimum (extrema) of fringe intensity, two-beam and multi-beam interferometric theory yield the same spacing. Therefore, at the extrema, the simpler two-beam theory is used. Tanaka et al. also vary the wavelength of the light being used in order to 1) get fringe extrema and, therefore, spacing measurement at different points along the slider; and 2) verify which order fringe is being detected.
The system of Tanaka et al. is limited in that it cannot measure spacing below .lambda./4 of the monochromatic light being used and it is too slow to measure air bearing resonances. Tanaka et al.'s system is clocked at a frequency of 15.8 kHz making it incapable of measuring typical air bearing resonances of 20 kHz or more.
Another method to measure slider/disk spacing is disclosed in Ohkubo et al.'s paper "Accurate Measurement of Gas-lubricated Slider Bearing Separation Using Visible Laser Interferometry" which was distributed as paper 87-Trib-23 by the American Society of Mechanical Engineers. This paper describes the basis of operation for the FM 8801 Laser-Based Flying Height Measuring System which is sold in the U.S.A. by ProQuip, Inc., Santa Clara, Calif. As described in Ohkubo et al.'s paper, the system uses a HeNe laser source. The beam from the laser goes through a beamsplitter where part of the beam is directed toward a reference photodetector which detects variation in the intensity of the laser source. The remaining part of the laser beam goes through a beam expander then through a lens which focuses the illumination onto the slider/glass disk interface. This illumination causes interference fringes which are focused onto a second measurement photodetector used for measuring intensity of the fringes. The measurement and reference signals from the two photodetectors are sent through amplifiers then into a divider circuit such that the interference signal is normalized to the input laser intensity. From the divider, the signal is sent through an A/D converter to a desk top computer for processing. The desk top computer digitizes interferometric intensity while the disk changes from a high speed to a low speed.
During the change of disk speed, Ohkubo et al. show flying height to decrease by more than .lambda./2. Since the flying height changes, the interferometric intensity varies enough to detect at least one maximum and one minimum fringe intensity. These maximum and minimum fringe intensities are recorded for reference. With the reference maxima and minima of fringe intensity, multi-beam interferometric theory is applied to determine spacing from intensity. However, since for a monochromatic interferometer intensity is a periodic function of spacing, the "fringe order" must be known to finally determine the spacing. This "fringe order" is precisely defined as the interval of spacing from n/4.lambda. to (n-1)/4.lambda. for n=1,2,3. Given an interferometric intensity, and a maximum and minimum fringe intensity for reference, the fringe order n must be determined in order to calculate spacing from the interferometric theory. According to Ohkubo et al.'s paper, the fringe order is determined by landing the slider on the disk by reducing disk speed while monitoring interferometric intensity. The fringe order can be determined by counting the number of times that the interferometric intensity rises to the maximum or falls to the minimum while the spacing is being reduced from the measurement point to the minimum spacing which is assumed to be the fringe order where n=1.
The Ohkubo et al. system has the following disadvantages: 1) the slider must have a design such that the flying height will increase to above .lambda./2 simply by changing disk speed; 2) the slider must be landed on the glass disk to determine the fringe order for the spacing calculation; and 3) at the points where the fringe intensity is a minima or maxima, the slope of the interferometric intensity/spacing curve becomes zero. At these points, the noise in electronic intensity measurement causes a large error in spacing measurement relative to the other spacings which are not directly on the fringe maximum or minimum.
The above-identified disadvantages may cause the following problems: 1) new sliders with a new geometries designed for very low flying height may not fly as high as .lambda./2, even at very fast disk speeds, so the Ohkubo et al. technique will not work for such; 2) landing the slider on the disk (required to determine the fringe order) may cause some damage to the air bearing surface of the slider. The possibility for damage to the air bearing surface during test is highly undesirable since many manufacturers test every slider assembly to insure proper flying height; and 3) the relatively high error in spacing measurement at fringe maxima and minima is hidden by "intensity correction" and data "smoothing". These procedures can introduce additional errors into the spacing which is finally calculated.
As magnetic recording technology continues to improve, slider flying heights should continue to decrease to below 100 nm. Also, some manufacturers are beginning to use fluid in the gap to permit smaller spacings. These, too, must be measured. The invention disclosed in this patent is intended to measure such flying heights, statically and dynamically, without having to land the slider on the disk, for have the spacing increase above .lambda./2 by only changing disk speed.
The method of intensity calibration and fringe order determination disclosed in this patent can also be applied to other measurements where spacing is decreased to the point of contact, in particular, a micro-hardness tester using a transparent probe could be implemented with interferometric measurement of spacing between the surface and the probe using this method of interferometric intensity calibration.